1. Field of the Invention
This invention generally relates to methods and systems for pattern failure discovery using comparisons of dies printed on a wafer with alternating failure modes for the wafer.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Inspection has always been an important part of fabricating semiconductor devices. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the devices to fail.
In general, semiconductor fabrication processes involve printing multiple dies on a wafer at the same process conditions across the entire wafer. For example, an entire wafer is typically processed with the best known process conditions such that the same devices can be formed across the entire wafer with the greatest probability that the devices will be formed with the desired characteristics. However, in some instances, multiple dies may be printed on a single wafer with different process conditions. Such printing of a wafer may be performed such that dies printed at different process conditions can be used to determine more information about a design for the wafer or the process performed on the wafer.
In one such instance, a wafer may be printed with different focus and exposure values across the wafer. Modulated dies (i.e., the dies printed at values of the focus and exposure other than nominal) may be compared to a nominal die (i.e., a die printed at nominal values for the focus and exposure) using what is commonly referred to as a process window qualification (PWQ) wafer layout. Alternatively, a modulated die may be compared to an adjacent, less or more, modulated die using a focus exposure matrix (FEM) wafer layout. In a FEM wafer layout, the dies across a row on the wafer may be varied in exposure, and the dies across a column on the wafer may be varied in focus. In this manner, different dies can be printed on the same wafer with different combinations of focus and exposure dose. PWQ is intended to increase the detection sensitivity by comparing modulated dies to a nominal die. In contrast, FEM is often used to define a process window by comparing adjacent modulated dies to each other.
The methods described above that use modulated dies have several challenges. For example, too many defects can be detected in the methods described above, which impairs the ability to effectively sample defects. In particular, when comparing highly modulated dies to a nominal die, detection is easier but at modulations near normal, the noise level can increase. This results in a substantially high defect count and often makes it difficult to sample real defects. If a defect is sampled from a lower modulation, the amount of pattern deformation may be so small as to be non-existent and therefore may be ignored as nuisance. Often, defect review time is wasted, and a defect review tool user may experience fatigue in looking at so many non-relevant patterns. In other words, much effort can be wasted in inspecting and/or reviewing at irrelevant lithography conditions. In addition, even with exhaustive efforts in defect review, key weak points still may be missed due to lack of ability to verify the defects using scanning electron microscope (SEM) images.
The currently used wafer layouts for the methods described above are not efficient for delivering data with relatively high signal for the purposes of the experiments. For example, the shot locations of the wafer area used for nominal die (at least two columns) are wasted since they are not being used to print potential systematic defects. In turn, users often find that there is an insufficient number of modulations across a wafer. On the other hand, printing one die per modulation reduces the chance of finding systematic defects due to variation across the wafer. In addition, if the modulated die happens to be at a “quieter” area of the wafer, pattern failure due to cross-wafer variation may be captured as well.
Accordingly, it would be advantageous to develop systems and/or methods that do not have one or more of the disadvantages described above.